A dichotomy in the nitration of fluoranthene with nitrogen dioxide/dinitrogen tetroxide: mechanistic and toxicological implications

Abstract

The nitration of fluoranthene with nitrogen dioxide can occur by two distinctive reaction pathways. These paths can be distinguished by product analysis, since fluoranthene is a nonalternant hydrocarbon. Free-radical nitration and electrophilic nitration give different products. In solvents with dielectric constants lower than that of CH{sub 2}Cl{sub 2} and in the absence of acid catalysis, the exclusive reaction pathway is homolytic in nature. The products of the homolytic reaction pathway can be interpreted as arising via a multiple-step addition-elimination mechanism and are notable for the formation of 2-nitrofluoranthene (the major product in CCl{sub 4} but absent under electrophilic nitration conditions) and the unusually large amounts of the expected 1,2-dinitrofluoranthene and 1,3-dinitrofluoranthene. The ionic reaction pathway is subject to both Lewis and Bronsted acid catalysis, particularly in CH{sub 2}Cl{sub 2}, and is inhibited by nonnucleophilic bases like 2,6-di-tert-butylpyridine. At temperatures lower than 25C, the ionic reaction pathway predominates, even in CCl{sub 4}. 2-Nitrofluoranthene is a marker for the free-radical nitration of fluoranthene, and its presence in polluted tropospheric air suggests that free-radical nitration by NO{sub 2} may occur under atmospheric conditions.